1. Field of the Invention
This invention relates to a process for preparing polyethylene terephthalate and its low molecular weight oligomers. More specifically, it relates to a process for preparing bis-(hydroxyethyl) terephthalate and its low molecular weight oligomers (hereinafter both of these will be referred to as "BHET") by the direct reaction of terephthalic acid and ethylene glycol.
2. Description of the Prior Art
Methods for preparing BHET by directly reacting terephthalic acid and ethylene glycol are known. Since terephthalic acid is not compatible with ethylene glycol, the reaction system is a solid-liquid heterogeneous system. When ethylene glycol is used in a relatively small proportion as compared with terephthalic acid, for example, in a proportion of not more than 2 moles per mole of terephthalic acid, the terephthalic acid becomes a wet mass or a slurry due to the effect of the liquid ethylene glycol and heat transmission is reduced. Consequently, a long period of time is required to elevate the temperature of the reaction system, and overheating occurs locally to cause undesirable side reactions. On the other hand, when the reaction is carried out using a large excess (e.g., more than 4 moles) of ethylene glycol, the flowability of the reaction system is improved, but another difficulty arises in that by-product diethylene glycol is formed in large quantities to lower the melting point of polyethylene terephthalate final product and deteriorate its thermal stability.
In an attempt to overcome these difficulties, Japanese Patent Publication 8915/57 discloses a method in which BHET is added in addition to terephthalic acid and ethylene glycol so that the flowability of the reaction system is improved by the melting of this oligomer. However, by this method side reactions are not sufficiently inhibited.
Japanese Pat. Publication No. 22463/71, on the other hand, discloses a method which comprises suspending or dissolving terephthalic acid in molten BHET, maintaining the suspension or solution at a temperature above the dew point of ethylene glycol, and feeding gaseous or liquid ethylene glycol into the suspension or solution to react it with terephthalic acid. Since a large quantity of ethylene glycol is fed in this method, the amount of ethylene glycol that distills off is large. Usually, the ethylene glycol that distills off contains diethylene glycol, terephthalic acid, BHET, mono-(hydroxyethyl) terephthalate, etc., and for the re-use of this ethylene glycol, it must be subjected to a purifying process such as distillation. In commercial operation, the equipment for this purpose is naturally of large scale, which results in increased production costs. In addition, thermal losses are high since it is necessary to gasify ethylene glycol in an amount greater than the stoichiometrical amount required for the reaction.
Japanese Patent Application (OPI) No. 94795/73 discloses a method in which the operating temperature is limited in order to more efficiently carry out the reaction shown in Japanese Patent Publication No. 22463/71. The method involves maintaining BHET at its boiling point in an equilibrium state at its average degree of polymerization, suspending or dissolving terephthalic acid in the BHET, and then adding ethylene glycol while controlling the amount added so that the boiling point of the reaction system can be maintained at the above mentioned temperature. This method makes it possible to shorten the reaction time and reduce the amount of by-product diethylene glycol. However, since no temperature control is exercised in a rectifying column, the distilling off of ethylene glycol cannot be completely prevented. Thus, as in the previously cited method, a purifying process is required, and this method is also not free from economic disadvantages.